Electroless copper plating on ceramic material



United States Patent 3,296,012 ELECTROLESS COPPER PLATING 0N CERAMTCMATERIAL Stewart G. Stalnecker, .lr., Raleigh, N.C., assignor to CorningGlass Works, Corning, N.Y., a corporation of New York No Drawing.Continuation of application Ser. No. 170,832, Feb. 2, 1962. Thisapplication Apr. 30, 1965, Ser. No. 452,333

6 Claims. (Cl. 11747) This application is a continuation of applicationSerial No. 170,832, filed February 2, 1962, now abandoned.

This invention relates to improvements in electroless plating orchemical deposition of copper on vitreous ceramic surfaces.

In applications such as, for example, terminating resistors comprisingan electroconductive film formed on a ceramic substrate, it has beenfound that adhesion of copper deposited by hereto-fore known electrolessor chemical deposition methods is poor. In addition, prior art methodsenabled the deposition of only very thin layers of copper which madepoor electrical contact to metallic or metallic oxide electroconductivefilms. The solderability of such layers was also poor. Furthermore,prior art plating solutions resulted in low copper yields because thesolutions became rapidly exhausted.

It is the specific object of this invention to provide an improvedmethod for electroless plating or chemical deposition of copper onceramic substrates which will overcome the above disadvantages.

It is another object of this invention to provide a method forelectroless plating of copper on ceramic material where the depositedcopper is firmly adhered to the surface.

A further object is to provide a method for electroless copper platingwhereby a comparatively thick layer of copper is deposited.

Still another object is to provide a method for electroless copperplating where the deposited copper makes good electrical contact withmetallic or metallic oxide electroconductive films.

A still further object is to provide an electroless copper platingsolution which has a comparatively long life.

Still another object is to provide an improved method for electrolessplating or chemical deposition of copper for economic resistortermination.

Additional objects, features and advantages of the present inventionwill become apparent, to those skilled in the art, from the followingdetailed description.

I have found that poor adhesion of copper films produced on ceramicsurfaces according to the prior methods is due to poor bonding of themetal to the ceramic caused by the lack of an effective anchorage in thesurface of the ceramic. To obtain a sufficiently strong anchorage, Ihave discovered that, instead of being merely roughened or chemicallyetched, the surface of the ceramic must be leached so as to provide itwith a submicroscopically porous surface.

Such leaching is accomplished in accordance with the invention byboiling the substrate in a suitable acid. The preferred acids vary withthe substrate material, for example, orthophosphoric acid is used withalumina, hydrofluoric acid is used with zircon, beryllia, steatite orsilicate glass-ceramics such as described in US. Patents No. 2,920,971and No. 2,968,578, and the like. A silicate glass-ceramic, as usedherein and as defined by the hereinabove noted patents, is a materialconsisting of a multiplicity of inorganic crystals dispersed in a glassymatrix where said crystals are formed by crystallization in situ from aglass consisting essentially of inorganic constituents of which themajor constituent is silica, and where said glassy matrix consistsessentially of the uncrystallized portion of the glass.

I have also found that poor solderability of copper films produced bythe prior methods is due to not only the poor adhesion of such films,but also to the thinness of such films. It has been impossible to obtaincomparatively thick copper films by employing conventional plat ingbaths because the baths rapidly become exhausted due to the rapidrecombination of copper ions and the subsequent precipitation ofmetallic copper in the bath, which results in the copper beingunavailable for plating long before a suitably thick copper film can bedesposited on the substrate. I have discovered that by adding a suitablesequestering or complexing agent to conventional plating bath solutions,recombination of copper ions can be significantly retarded, therebypreventing the precipitation of metallic copper in the plating bath andenabling substantially greater amounts of the copper in the bath to beavailable for plating for substantially greater periods of time.

In accordance with conventional electroless copper plating methods, asubstrate is suitably cleaned and then sensitized by successiveimmersions in an aqueous solution of catalytic metal and an aqueoussolution of a reducing agent such as, for example, solutions of cupricammonium hydroxide and sodium borohydride, or the like, and rinsing inwater following each such immersion. Thereafter, the substrate isimmersed in the plating. bath comprising, for example, an aqueoussolution containing a salt of copper and an alkali metal hydroxide inpresence of salts such as potassium sodium tartrate, and/or sodiumcarbonate. After immersion for a period of about 15 to 30 minutes thebath is substantially exhausted and a film of copper ranging from about0.000005 inch to about 0.00002 inch in thickness has been deposited onthe substrate.

The instant invention may be carried out using any conventionalelectroless copper plating bath solution. An example of one suitablebath is as follows. A concentrate is prepared comprising 79 g. potassiumsodium tartrate, 28 g. copper sulfate, 20 g. sodium hydroxide and 32 g.sodium carbonate per liter of distilled water. The plating bath is thenprepared by mixing by volume 2 parts of said concentrate with 1 partdistilled water and adding 1-6 cc. formaldehyde per 100 cc. of saidconcentrate.

A typical example of carrying out the method of this invention isillustrated by the following. A 5 inch square substrate of hot pressedalumina having a thickness of about 0.010 inch was suitably cleaned bydipping in an ultrasonically agitated acetone bath. The aluminasubstrate was then placed in a leaching bath of boiling ortho-phosphoricacid and allowed to boil for about 3 minutes. It has been found thatleaching is satisfactorily accomplished if the ceramic is maintained inthe leaching bath for a period ranging from at least 1 minute to about 5minutes, although leaching for longer periods is possible if any partialpreplating or precoating of the substrate is not thereby adverselyaffected. The device was then rinsed in water, placed in a solution ofabout 5% hydrogen peroxide for about 5 minutes, then boiled in water forabout 10 minutes and thereafter air dried.

The alumina substrate was then sensitized by immersing in a 3.0%stannous chloride solution for about 3 minutes, rin ing in water,immersing in a 0.01% palladium chloride solution for about 3 minutes andfinally thoroughly rinsing in water.

A conventional electroless copper plating bath such as hereinabovedescribed was prepared to which was added a sequestering or complexingagent, tetrasodium salt of ethylenediaminetetraacetic acid in the amountof 2 parts per parts by volume of the plating bath concentrate. It hasbeen found that satisfactory results may be obtained if the sequesteringor complexing agent is added t the plating bath in amounts ranging fromabout 1 part to parts by volume per 100 parts of the plating bathconcentrate, or at least 0.5% by volume of the plating bath.

The alumina substrate was placed in the plating bath containing thesequestering or complexing agent and maintained therein for about 3hours. It was found that a well adhered copper coating of about 0.00075inch thickness wa deposited upon the alumina. It was also found thatthis'copper coating was readily solderable.

Copper may also be selectively deposited on a substrate by the method ofthis invention. In applications such as resistor termination where anelectroconductive coating resistance element is formed on a substrate,the herein described method may be used for forming said resistortermination by suitably masking the portions of the resistance devicewhere the copper plating is not wanted.

Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended thatsuch details be limitations upon the scope of the invention exceptinsofar as set forth in the following claims.

I claim:

1. In a method of chemically depositing copper on a' surface of analumina substrate comprising the steps of cleaning said substrate,sensitizing it, rinsing it with water and immersing it in a platingbath, the improvement comprising the steps of making the surface of saidalumina substrate submicroscopically porous by leaching after saidcleaning step in a bath comprising about 85% aqueous solution ofortho-phosphoric acid,

sensitizing the leached surface by successively immersing said substratein an aqueous solution of stannous chloride and an aqueous solution ofpalladium chloride for about 3 minutes each, and then immersing saidsubstrate in a plating bath containing a salt of copper to which bathtetrasodium salt of ethylenediarninetetraacetic acid has been added inthe amount of at least 0.5% by volume.

2. The method of chemically depositing a strongly adherent layer ofcopper on the surface of a ceramic substrate comprising the steps offorming a submicroscopically porous surface on said substrate byleaching in an acid bath for at least 1 minute,

rinsing the leached surface with water,

sensitizing the leached surface by successively immersing said substratein an aqueous solution of catalytic metal and an aqueous solution of areducing agent for about 3 minutes each,

rinsing said substrate with water, and thereafter immersing saidsubstrate in a plating bath containing a salt of copper to which bathtetrasodium salt of ethylenediaminetetraacetic acid has been added inthe amount of at least 0.5 by volume.

3. An article having copper deposited on at least part of its surface bythe method of claim 2.

4. The method of chemically depositing a strongly adherent layer ofcopper having a thickness of at least 0.00002 inch on the surface of aceramic substrate comprising the steps of forming a submicroscopicallyporous surface on said substrate by leaching in an acid bath for atleast 1 minute,

rinsing said substrate with water,

placing said substrate in a solution of 5% hydrogen peroxide for about 5minutes,

boiling said substrate in water,

air drying said substrate,

sensitizing the leached surface by successively immersing said substratein an aqueous solution of stannous chloride and an aqueous solution ofpalladium chloridefor about '3 minutes each,

rinsing said substrate with water, and thereafter immersing saidsubstrate in a plating bath comprising an aqueous solution of a salt ofCopper, an alkali metal hydroxide and tetrasodium salt ofethylenediaminetetraacetic acid for at least 20 minutes, saidtetrasodium salt being present in an amount effective to retardprecipitation of metallic copper in said plating bath.

5. The method of chemically depositing a strongly adherent layer ofcopper having a thickness of at least 0.00002 inch on the surface of analumina substrate comprising the steps of forming a submicroscopicallyporous surface on said substrate by leaching in an aqueous solution ofortho-phosphoric acid for at least 1 minute, rinsing said substrate withwater,

placing said substrate in a solution of 5% hydrogen peroxide for about 5minutes,

boiling said substrate in water,

air drying said substrate,

sensitizing the leached surface by successively immersing said substratein an aqueous solution of stannous chloride and an aqueous solution ofpalladium chloride for about 3 minutes each,

rinsing said substrate with water, and thereafter immersing it in aplating bath comprising an aqueous solution of a salt of copper, analkali metal hydroxide and tetrasodium salt ofethylenediaminetetraacetic acid for at least 20 minutes, saidtetrasodium salt being present in the amount of at least 0.5% by volume.

6. The method of chemically depositing a strongly adherent layer ofcopper having a thickness of at least 0.00002 inch on the surface of asilicate glass-ceramic substrate comprising the steps of forming asubmicroscopically porous surface on said substrate by leaching in anaqueous solution of hydrofiuoric acid for at least 1 minute,

rinsing said substrate with water,

placing said substrate in a solution of 5% hydrogen peroxide for about 5minutes,

boiling said substrate in water,

air drying said substrate,

sensitizing the leached surface by successively immersing said substratein an aqueous solution of stannous chloride and an aqueous solution ofpalladium chloride for about 3 minutes each,

rinsing said substrate with water, and thereafter immersing saidsubstrate in a plating bath comprising an aqueous solution of a salt ofcopper, an alkali meal hydroxide and tetrasodium salt ofethylenediaminetetraacetic acid for at least 20 minutes, saidtetrasodium salt being present in the amount of at least 0.5% by volume.

References Cited by the Examiner UNITED STATES PATENTS 2,690,402 9/1954Crehan 11754X 2,872,312 2/1959 Eisenberg ll7-l60 2,956,901 10/1960Carlson 117-160 X 2,968,578 1/1961 Mochel 11754 3,042,566 7/1962 Hardy1562 3,075,856 1/1963 Lukes 117-47 3,093,509 6/1963 Wein 1171603,095,309 6/1963 Zeblisky et al. 117130 3,119,709 1/1964 Atkinson 11747FOREIGN PATENTS 45,696 9/1952 India.

ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner.

1. IN A METHOD OF CHEMICALLY DEPOSITING COPPER ON A SURFACE OF ANALUMINA SUBSTRATE COMPRISING THE STEPS OF CLEANING SAID SUBSTRATE,SENSITIZING IT, RINSING IT WITH WATER AND IMMERSING IT IN A PLATINGBATH, THE IMPROVEMENT COMPRISING THE STEPS OF MAKING THE SURFACE OF SAIDALUMINA SUBSTRATE SUBMICROSCOPICALLY POROUS BY LEACHING AFTER SAIDCLEANING STEP IN A BATH COMPRISING ABOUT 85% AQUEOUS SOLUTION OFORTHO-PHOSPHORIC ACID, SENSITIZING THE LEACHED SURFACE BY SUCCESSIVELYIMMERSING SAID SUBSTRATE IN AN AQUEOUS SOLUTION OF STANNOUS CHLORIDE ANDAN AQUEOUS SOLUTIO OF PALLADIUM CHLORIDE FOR ABOUT 3 MINUTES EACH, ANDTHEN IMMERSING SAID SUBSTRATE IN A PLATING BATH CONTAINING A SALT OFCOPPER TO WHICH BATH TETRASODIUM SALT OF ETHYLENEDIAMINETETRAACETIC ACIDHAS BEEN ADDED IN THE AMOUNT OF AT LEAST 0.5% BY VOLUME.